Chemistry Reference
In-Depth Information
Fig. 5 Higher nuclearity organothiolato-gold cluster compounds. Reproduced with permission
from Zeng and Jin [
32
]
superatom structure, and this aspect will be discussed in more detail in the bonding
section below. Until 2007 [Au
39
(PPh
3
)
14
Cl
6
]
2+
, which has a close-packed core of
gold atoms stabilised by phosphine and chloro-ligands, represented the highest
nuclearity gold cluster compound [
21
]. The metal framework comprises of layers
of hexagonally close-packed gold atoms which down the threefold symmetry axis
follow the sequence 1:9:9:1:9:9:1. Only one gold atom is completely encapsulated
and resides at the centre of a hexagonal antiprism and has a coordination number of
12. The Au-Au bonds radiating from this gold average 3.04
, which surprisingly is
not shorter than the tangential bonds Au-Au bonds. Eighteen of the surface gold
atoms are sufficiently sheltered that they do nor form bonds to the 20 ligands.
The recent synthesis and characterisation of high nuclearity gold clusters
stabilised by organothiolato-ligands (SR) has demonstrated that the central kernels
consist of a close-packed polyhedron based on either icosahedra or cuboctahedra.
[Au
25
(SR)
18
]
is based on an icosahedral kernel, [Au
38
(SR)
24
] on a pair of icosa-
hedra sharing a face and [Au
102
(SR)
44
] on a decahedron of 79 atoms with fivefold
symmetry [
32
,
33
]. In contrast [Au
36
(SR)
24
] has a cuboctahedral kernel and
[Au
28
(SR)
20
] is based on two cuboctahedra with shared faces (see Fig.
5
).
A model which accounts for the structures and stoichiometries of these clusters
presents a considerable challenge to theoreticians and clearly a model which works
for both classes of clusters would provide an important unifying feature. It is also
Å
